Generally, portable devices such as mobile phones and portable gaming devices employ liquid crystal display devices as display units thereof. Because mobile phones and the like are battery-powered, and low power consumption is strongly demanded, information that needs to be displayed constantly such as a time clock or a battery level is displayed on a reflective sub-panel. In recent years, there has been a demand for performing both a normal display with a full-color display and a reflective constant display in a single main panel.
FIG. 35 shows an equivalent circuit of a pixel circuit of a typical active matrix type liquid crystal display device. FIG. 36 shows a circuit arrangement example of an active matrix type liquid crystal display device with m×n pixels. Both m and n are integers of 2 or greater.
As shown in FIG. 36, switching elements made of thin film transistors (TFTs) are provided at respective intersections of “m” number of source lines SL1, SL2, . . . , SLm and “n” number of scanning lines GL1, GL2, . . . , GLn. In FIG. 35, the respective source lines SL1, SL2, . . . , SLm are represented by source lines SL. Similarly, the respective scanning lines GL1, GL2, . . . , GLn are represented by scanning lines GL.
As shown in FIG. 35, a liquid crystal capacitance element Clc and an auxiliary capacitance element Cs are connected in parallel through a TFT. The liquid crystal capacitance element Clc is made of a laminated structure that includes a pixel electrode 20, an opposite electrode 80, and a liquid crystal layer sandwiched therebetween. The opposite electrode is also referred to as a common electrode.
In FIG. 36, only TFTs and pixel electrodes (black rectangular portions) are illustrated in the respective pixel circuits for simplification.
The auxiliary capacitance Cs has one end (one electrode) connected to the pixel electrode 20, and the other end (the other electrode) connected to an auxiliary capacitance line CSL so as to stabilize a pixel data voltage maintained in the pixel electrode 20. The auxiliary capacitance Cs is provided so as to suppress a change in electrical capacitance of the liquid crystal capacitance element Clc between a black display and a white display, which occurs due to a leakage current of the TFT and dielectric constant anisotropy of liquid crystal molecules. It can also prevent the pixel data voltage held by the pixel electrode from changing due to a voltage change and the like caused by a parasitic capacitance between the pixel electrode and neighboring wiring lines. By sequentially controlling a voltage applied to the scanning lines, TFTs connected to a particular scanning line are turned on, and for every scanning line, voltages that correspond to pixel data are supplied to the respective source lines, and are written in the corresponding pixel electrodes.
In the normal display mode with full-color display, even when a displayed image is a still image, the same display content is repeatedly written in the same pixel in every frame. By refreshing the voltage of the pixel data held by the pixel electrode in this way, the change in pixel data voltage is minimized, thereby ensuring a high-quality display of a still image.
The power consumption for driving a liquid crystal display device is mostly accounted for by the power used by a source driver to drive source lines, and is substantially represented by a relational expression shown in Formula 1 below. In Formula 1, P is power consumption, “f” is a refresh rate (the number of refresh operations for one frame executed per unit time), C is a load capacitance driven by the source driver, V is a driving voltage of the source driver, “n” is the number of the scanning lines, and “m” is the number of the source lines, respectively. The refresh operation refers to an operation of applying voltages to pixel electrodes through source lines while maintaining display content.P∝f·C·V2·n·m  Formula 1
On the other hand, in the constant display, because the display content is a still image, the pixel data voltage does not necessarily have to be refreshed for every single frame. Therefore, in order to further reduce the power consumption of the liquid crystal display device, the refresh frequency could be reduced in the constant display. However, when the refresh frequency is reduced, the pixel data voltage held by the pixel electrode is changed due to the leakage current of the TFT. This voltage change causes the display brightness (liquid crystal transmittance) of each pixel to change, which is observed as flickering. Also, because the average potential in each frame period is lowered, degradation of the display quality such as an insufficient contrast may occur.
As a technique to solve the problem of the display quality degradation caused by the reduced refresh frequency in the constant display of still images, such as the battery level, the clock display, and the like, and to reduce the power consumption at the same time, a configuration described in Patent Document 1 below has been disclosed, for example. The configuration disclosed in Patent Document 1 is capable of performing both a transmissive liquid crystal display and a reflective liquid crystal display. Further, pixel circuits in a pixel region that can perform the reflective liquid crystal display are provided with memory units, respectively. This memory unit stores a voltage signal indicative of information to be displayed in the reflective liquid crystal display section. When the reflective liquid crystal display is performed, the pixel circuit draws the voltage stored in the memory unit, thereby displaying information corresponding to the voltage.
In Patent Document 1, the memory unit is SRAM that allows the voltage signal to be stored statically. This eliminates the need for the refresh operation, and therefore, maintenance of display quality and reduction in power consumption can be achieved at the same time.